Nonwoven fabric as obtained is generally wound into roll form to be transported or stored and unrolled to be processed into a desired form for use as a material for producing disposable diapers, cleaning sheets, and the like. A sheet in roll form decreases in bulkiness due to great winding pressure. There has been proposed a method for restoring bulkiness of nonwoven fabric whose bulk has been reduced under winding pressure as disclosed in patent literature 1 (see below).
The assignee common to this patent application proposed in patent literature 1 a method for restoring bulkiness of nonwoven fabric which contains crimped the thermoplastic fiber and is in a roll form. The method includes unwinding the nonwoven fabric from a stock roll and blowing hot air at a temperature lower than the melting point of the thermoplastic fiber to the unwound nonwoven fabric to make the nonwoven fabric increase in bulkiness.
A hot air heating apparatus capable of controlling the heating temperature is proposed in patent literature 2 below, in which the temperature fluctuation of the hot air blown to a substrate is minimized.
Patent literature 2 discloses a system composed of hot air heating units. Each unit includes a casing having therein a fan for air circulation, a heater, tubular nozzles, and hot air guide plates. The tubular nozzle has therein equally spaced baffle plates so as to blow hot air from its slit uniformly onto a substrate.